Please read my post and please refrain from telling me the obvious...
The file names have nothing to do with the type of builtin subtilte format that Matroska file can have.
I know the subtitles are 'inside' a .MKV file, but they have a format. They can be SRT, SMI, IDX/SUB, etc. You don't see them as a separate file, they are builtin, but if the format is IDX/SUB, you need for example the VSFilter. I can't reproduce your problem, perhaps it happens only with a particular subtitle format.
So... do you know the format of the builtin subtitles in your files?
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